(1) Field of the Invention
The present invention relates to an amplifier circuit, and more particularly to a radio frequency amplifier circuit (hereinafter referred to as an "RF amplifier circuit") for dividing a radio frequency input signal into an analog and a differential radio frequency output signal.
(2) Description of the Related Art
A conventional RF amplifier circuit has a circuit configuration, as shown in FIG. 1, wherein a single input signal is amplified at an input stage consisting of an emitter grounded transistor 21 and an output stage consisting of self-biased Darlington-connected pair transistors 22, 23 with local shunt and series feedback. Another conventional RF amplifier circuit has a circuit configuration, as shown in FIG. 2, wherein an input signal is amplified by Darlington-connected pair transistors 22', 23' with resistive bias network and shunt and series resistive feedback. Such circuits are disclosed in, for example, the "SILICON BIPOLAR FIXED AND VARIABLE GAIN AMPLIFIER MMICs FOR MICROWAVE AND LIGHTWAVE APPLICATIONS UP TO 6 GHz", 1989 IEEE MTT-S Digest, Pages 109-112. Generally, where an RF amplifier circuit is constituted by discrete elements, in order to bring out the characteristics of the transistors used to their maximum extent, it is usual for the input-output terminals to be provided with matching circuits which match the input-output impedances of the transistor circuit. The circuits shown in FIGS. 1 and 2 are ones in which a self-biasing method is applied and a negative feedback technique is provided so as to be suited in the realization of LSIs.
Further, an example of a conventional circuit which is more suited to the realization of LSIs is a differential amplifier circuit, as shown in FIG. 3, wherein two input signals processed by two-stage emitter follower circuits 25, 27 and 26, 28, respectively, are amplified by a differential amplifier circuit formed by a pair of differential transistors 1, 2, a constant-current source 3 and load resistors 4, 5. This circuit is disclosed in the "Multi-Gigabit-Per-Second Silicon Bipolar IC's for Future Optical-Fiber Transmission Systems". IEEE Journal of Solid-State Circuits, Vol. 23 No. 3 June, 1988, Pages 664-675. This kind of differential amplifier circuit is widely used in LSIs because the characteristics of the differential pair transistors 1 and 2 can be coincided with each other.
As a specific example wherein a single input signal and a differential input signal are used in combination. FIG. 4 shows a frequency converter using a phase-locked loop (PLL) synthesizer. There is provided a PLL loop which is formed by having an output signal of a voltage controlled oscillator (VCO) 10 dividedly inputted to a frequency divider (DIV) 12 and to a local amplifier 16 through a buffer 11 and an attenuator 41 and which feeds back an output of the frequency divider 12 to the voltage controlled oscillator (VCO) 10 as a control signal by a frequency-phase comparator or phase detector (PFD) 14. A local signal (LOCAL) from the local amplifier 16 is supplied to a mixer 15 to which an RF signal is also inputted and which outputs an intermediate frequency signal (IF) in the receiver system. Generally, in the ICs, the above frequency divider (DIV) 12 is realized by a differential circuit and the local amplifier 16 is realized by a single-input amplifier circuit of an emitter grounded type. In order to realize the above overall arrangement by use of discrete elements, the attenuator 41 consisting of a resistor network as shown in FIG. 4 is normally used.
In each of the conventional amplifier circuits shown in FIGS. 1-3, it is possible to obtain only either a single output or a differential output. Therefore, it is necessary to have the signal divided into two kinds of signals for forming the system as shown in FIG. 4, which requires a discrete circuit to be provided externally of the IC, resulting in a characteristic problem of causing the attenuation of signals due to the presence of the divider circuit. This is a problem to be solved in the conventional circuits.
Further, the amplifiers may possibly be formed all in a semiconductor integrated circuit since such circuits are based on a differential circuit suited to LSIs. However, the problem therein is that, since the power efficiency is inferior so that, if a desired output power is to be obtained through low impedance line, for example, 50 .OMEGA. transmission line, the current consumption is larger than in the amplifier circuit dealing with a single signal. This is another problem to be solved in the conventional circuits.